Areal density of magnetic recording in hard disc data storage devices is a product of track density and bit recording density. Drive storage capacity can be increased without increasing the number or size of recording discs by increasing areal density, such as by increasing track and/or bit density. Track density can be increased by decreasing track width and/or spacing between tracks. Bit density can be increased by increasing the number of bits per inch recorded along the length of each track, such as by increasing data frequency for the track. However, there are practical limits to increasing bit density. More particularly, increasing the track and/or bit density decreases the signal-to-noise ratio of the recovered data signals on readback, leading to unacceptably increased data errors.
Bit density is often expressed as the number of bits per inch (BPI) along the length of a track. Because the tracks on a recording disc are circular and are concentrically arranged, the length of a track at an outer radius is significantly greater than the length of a track at an inner radius. Consequently, for a given recording frequency, bit density is greater at an inner radius than at outer radius. Thus, the BPI of each track is related to the data frequency and the radial location of the track.
Ordinarily, disc drives are manufactured to meet some set of specifications that define the performance of the drive. One element of the specifications relates to the bit error rate (BER). The bit error rate is related to bit density and is a measure of the performance of the head/medium combination in recording and recovering data. The BER is expressed as a number of recorded bits successfully recovered per error (e.g., 10X). By convention, the bit error rate is expressed simply as X, so a bit error rate of 7.8 is equivalent one error in 107.8 recovered bits. Thus greater BERs represent better head/medium performance.
Typically, increases in bit density (BPI) adversely affects the error rate, reducing the BER number. More particularly, higher BPIs require more closely packed data bits along the length of the track, thereby degrading the signal-to-noise ratio and decreasing the BER number. Therefore, one goal of disc drive performance is to establish the BPI to meet a specified minimum BER.
One well-known technique to increase bit density is to segment the disc surface into radial zones and record data at higher frequencies in outer zones. This is known as “zone bit recording” and results in a bit density that is substantially the same in each radial zone. Consequently, the BER is substantially the same over the entire disc, and within stated specifications.
However, head/disc performance is not equal for all discs of a storage medium. Manufacturing variances of both the heads and the discs often result in slight variances of the BER between head/disc combinations of a multi-surface storage device. Consequently, an improvement to zone bit recording, particularly useful in disc drives having plural recording discs (i.e., disc packs), is to adjust the BPI for each disc to an optimal value based upon a target BER. This concept, known as variable bits per inch (VBPI) selects a recording frequency for each disc based on the data transfer capability of the particular head/disc combination.
In practice, the BER might vary between zones for a given head/disc combination having a given BPI. Manufacturing tolerances may result in slight differences in performance of a head/disc combination over the various zones of the disc, resulting in different BERs between zones. As a result, some disc drives failed qualification tests at manufacture simply because one or a few zones failed the BER test at the established BPI. An examination of the failed disc drives revealed that further adjustment of the BPI of the failed zones and of other passing zones could result in a passing disc drive.
However, it has not been economically feasible to test the BER of each zone of each disc surface of each disc drive manufactured to optimize the BPI. Therefore, there is a need for a technique to select the optimal data frequency that meets a minimal BER threshold without requiring an extensive test of each zone of each disc for each copy of the model of disc drive. The present invention provides a solution to this and other problems, and offers other advantages over the prior art.